This invention relates to a stabilized thermoplastic resin composition, a method to synthesize the composition and articles made from the compositions.
Polycarbonate is a useful engineering plastic for parts requiring clarity, high toughness, and, in some cases, good heat resistance. However, polycarbonate also has some important deficiencies, among them poor chemical and stress crack resistance, poor resistance to sterilization by gamma radiation, and poor processability. Blends of polyesters with polycarbonates provide thermoplastic compositions having improved properties over those based upon either of the single resins alone. Moreover, such blends are often more cost effective than polycarbonate alone. The miscibility of PC with the polyesters gives the blends the clarity needed, but this is restricted to (semi)aliphatic polyesters such as poly(cyclohexane dimethanol cyclohexane dicarboxylate) (PCCD) or a glycolized copolyester such as polyethylene glycol cyclohexane dimethanol terephthalate (PCTG). PCT patent application no. WO 02/38675 discloses a thermoplastic composition comprising PC, PCCD, and an impact modifier.
U.S. Pat. Nos. 4,188,314, 4,125,572; 4,391,954; 4,786,692; 4,897,453, and 5,478,896 relate to blends of an aromatic polycarbonate and poly cyclohexane dimethanol phthalate. U.S. Pat. No. 4,125,572 relates to a blend of polycarbonate, polybutylene terephthalate (PBT) and an aliphatic/cycloaliphatic iso/terephthalate resin. U.S. Pat. No. 6,281,299 discloses a process for manufacturing transparent polyester/polycarbonate compositions, wherein the polyester is fed into the reactor after bisphenol A is polymerized to a polycarbonate.
Moldable crystalline resin compositions such as polycarbonate-polyester blends are desirable for many applications. On exposure to high temperature and humidity, such blends may exhibit relatively poor hydrolytic stability. Another problem associated with these blends is due to ester-carbonate interchange, also known as trans esterification, which may lead to loss of mechanical properties. Catalyst quenchers are typically used to prevent such interchange reactions. However these catalyst quenchers can also promote degradation of polymer chains and contribute to decrease in hydrolytic stability.
Conventionally phosphorus derivatives such as phosphoric acid, phosphates have been used as quenchers. U.S. Pat. Nos. 4,532,290, 4,555,540, 4,401,804, U.S. Pat. No. 20,030,032,725, describes the phosphorous-containing compounds include phosphoric acid, certain organic phosphorous compounds such as distearyl pentaerythritol diphosphate, mono or dihydogen phosphate are useful in deactivating metallic catalyst residues. The use of phosphite stabilizers is not satisfactory because of the tendency to be unstable to both hydrolysis and oxidation. U.S. Pat. No. 4,452,933 teaches the use of hydroxy or amino substituted carboxylic acid derivatives such as Methyl salicylate, Malic acid, Glycine or dibutyl tartrate to effectively inhibit ester-carbonate interchange reaction. The U.S. Pat. No. 4,560,722 discloses a stabilized polycarbonate polyester blend with boric acid as a stabilizer. EP Patent 02 72417 teaches the use of polyols as a color stabilizer stabilizing the polycarbonate polyester composition.
U.S. Pat. No. 5,087,665 Chung et al. disclose a method of improving the hydrolytic stability of blends of polycarbonate and polyethylene terephthalate, by adding polyethylene to the blends. U.S. Pat. Nos. 5,411,999 and 5,596,049 describe the use of epoxy based material in conjugation with the catalyst quenchers to promote hydrolytic stability. However, a disadvantage is that the epoxy compounds were used in combination with metal catalyst, such as sodium stearate, which in turn may result in loss in polycarbonate molecular weight. U.S. Pat. No. 4,760,107 teaches a addition of a combination of an epoxide with polyols to polycarbonate polyester blends for color retention properties. European Patent Nos. EP 0 273149 and EP 0 497 818, describe additions of epoxy oligomeric materials to certain polyesters, disclose thermal stability in glass reinforced and/or flame-retarded polyester formulations. U.S. Pat. No. 5,300,546 relates to polyester compositions with mineral fillers giving a ceramic feel which have improved hydrolytic stability and melt viscosity stability.
There is a continuing need for polycarbonate polyester blends having a good balance of optical property, processability, solvent resistance and hydrostability in addition to good mechanical and thermal properties.